This invention relates to a method of selecting a product to be inspected and a program for selecting a product to be inspected by the product sampling inspection conducted in the product production process.
In a product such as a chemical product or plastic product produced through the process of mixing a plurality of materials, the characteristic values of the product functions and the content of the chemical substances in the product may be varied depending on the production conditions including the processing temperature and the processing time. Therefore, the quality of the product to be shipped is required to be secured by conducting product inspection during the production process. The method of the product inspection includes a total inspection and a sampling inspection. The total inspection is the one conducted on all the products produced (in all units of product lots, for example). In the sampling inspection, on the other hand, samples are picked out at predetermined intervals from the products flowing on the production line and inspected, and the result is used to represent the result of product inspection for the particular intervals.
The total inspection, though recommended for securing product quality, poses the problem that the inspection steps and the inspection cost are increased. For the products and materials of a quality stabilized to a certain degree, therefore, the sampling inspection is conducted.
The conventional sampling inspection, as described in JP-A-9-269294, is conducted for each predetermined number of products or product lots at an optimum frequency set according to the balance between the product quality to be secured and the inspection cost. The (product) lot is defined as a group of products processed at the same time in each production process. According to this technique, the amount of the loss incurred as the result of overlooking a defect for a predetermined inspection frequency is estimated using the relation between the rate at which a product defect is detected by an inspection device and the frequency at which a product defect occurs, and the inspection frequency is optimized in such a manner as to minimize the sum of the loss amount and the inspection cost required for the inspection conducted at the particular frequency.
The result of this product inspection is utilized to investigate the cause of the defect as well as to secure the product quality as described below. A product defect is considered to be caused either by the material used for production (hereinafter referred to as “the material-derived defect”) or by the production equipment or the production conditions (hereinafter referred to as “the production-derived defect”).
A material-derived defect is investigated by measuring the amount of the chemical substances, for example, contained in a sample of a material lot used in the defective product. In an anomalous case where the investigation result indicates that the amount of a chemical substance is larger than a specified value, the particular material lot is determined as the cause of the defect, and the shipment of the other products produced from the particular material lot is suspended or the products are recalled. A production-derived defect, on the other hand, is investigated by checking whether the production history of the equipment, etc. used in the production of the defective product is anomalous or not. The “production history” is defined as the chronologically recorded information including the materials used for production in a particular piece of equipment, the lots of a product in process and the production conditions such as the processing temperature and the processing time.
The investigation of a material-derived defect is a time-consuming job requiring the preparation of an analysis sample and the waiting time before reaction. In view of this, a material lot used for a product lot is not investigated as long as the particular product is found not to be defective by the sampling inspection conducted during the production, on the assumption that the possibility is low that the particular material lot causes the defect even in the case where the particular product is found defective after shipment.
The conventional sampling inspection method used for this investigation of the cause of the defect poses the problem described below.
In the case where a product is found defective after being shipped, it is important to specify the cause of the defect quickly. A product lot having a smaller number of material lots requiring the investigation (hereinafter sometimes referred to as the investigation-required material lots) at the time of defect occurrence requires a correspondingly smaller number of steps to trace the cause of the defect, and therefore, the cause of such a defect can be specified at an early time. In the sampling inspection of a product in the production process, therefore, it is important to select a product lot requiring the inspection (hereinafter sometimes referred to as the inspection-required product lot) in such a manner as to minimize the material lots to be investigated at the time of defect occurrence.
The conventional inspection method in which product lots are sampled at predetermined intervals, however, fails to take the material lots used for the product into consideration, and therefore, poses the problem that a multiplicity of material lots may be required to be investigated at the time of defect occurrence.